Process of coating austenitic steel with chromium alloy coatings



United States Patent 3,053,689 Patented Sept. 11, 1962 This invention relates to metal articles having hard, high Wear and oxidation resistant alloy coatings and a method for producing the same. More particularly this invention relates to internal combustion exhaust valves and the like and a method of applying a hard, high wear and oxidation resistant cobalt-chromium type alloy coating thereto.

With the advent of the high compression internal combustion engine and the more extensive use of leaded fuels in recent years, the exhaust valves of internal combustions engines have been subject to rapid oxidation and high temperature corrosion by the hot combustion gases. The gases which result from the burning of leaded gasolines, the lead compounds of which at least partially decompose during fuel combustion, are especially deterimental and greatly shorten valve life. Consequently, exhaust valve failure frequently occurs either because of high temperature corrosion of the valve seating face or because of corrosion of the stem immediately beneath the valve head. Severe corrosion of this stem results in its necking down to a point where the stem will fracture.

To alleviate the aforementioned oxidation and wrrosion problems and to provide poppet valves which possess a greatly increased operating life, these valves may be coated with a hot hard high wear and oxidation resistant alloy.

It is already the practice to coat the seating surfaces of exhaust valves with a cobalt-chromium alloy, such as the well known Stellite alloys, by melting the rod of the alloy and allowing the molten alloy to drop onto the seating surface of the valve While the latter is heated and caused to rotate, to cause the drops of molten alloy to coalesce and form welded coating on the seating surfaces. It has also been proposed to spray the valve head with particulate cobalt-chromium alloy and thereafter to heat the valve head surface sufliciently to melt the particulate coating and effect a fusion of the coating metal and the valve head surfaces. It is also known to place an annular ring of the cobalt-chromium alloy over the area of the valve head to be coated and then to heat the valve head surface and the annular ring sufficiently to melt the latter and effect a fusion of the metals. Insofar as it is known all of the known methods involve the deposition of excessive quantities of the cobaltchromium alloy over the valve surfaces and/or a heating of the valve head surfaces to substantially the softening point thereof, whereby the valve head surfaces may be distorted since the melting temperatures of conventionally used exhaust valve steels are in the same vicinity as the melting temperatures of the cobalt-chromium alloys. As a result, the valves must be subjected to extensive grinding or other finishing operations.

Among the objects of this invention is to provide an im proved method of economically and efliciently applying a hard, high temperature wear and oxidation resistant cobalt-chromium base alloy to an exhaust valve surface which does not require that either the valve base metal or the cobalt-chromium alloy be heated to a melting or softening point and which involves a minimum finishing operation. A simplified flow diagram of the method is as follows:

austenitic steel article spray Ni-Or alloy coating spray Oo-Or alloy coating heat to melt Ni--Or alloy layer only and form fusion bond In general, these and other objects are accomplished by first applying in particulate form, as for example by spraying, a first coating of a relatively low melting point, hard, wear and oxidation resistant alloy which is essentially a nickel-chromium alloy including minor proportions of carbon, silicon and/ or boron, then applying a second coating in particulate form, as by spraying, of a relatively high melting point, high temperature, hard, oxidation and wear resistant alloy which is essentially a cobalt-chromium alloy including minor proportions of tungsten and carbon and thereafter subjecting the composite structure to heat sufficient to melt the intermediate nickel-chromium alloy and bond the cobalt-chromium alloy coating to the valve base metal through the nickel-chromium alloy coating without softening or melting the valve base metal.

Further objects and features of the invention will be apparent from a reading of the following description of the invention and the claims.

Most of the exhaust valves presently in use are made of what is known as Austenitic valve steels. Representative chemical compositions of these steels are the following:

Silchrome X10 Percent Carbon .3 to .45. Manganese .8 to 1.3. Phosphorous .03. Sulphur .03 max. Silicon 2.75 to 3.25. Chromium 18 to 20. Nickel 7.5 to 8.5.

Balance essentially Iron Silchrome X l 0-N This alloy is the same as the silchrome X10 alloy above except that it includes .15 to 25% nitrogen.

Balance essentially Iron.

TXCR

Percent Carbon .3 to .4. Manganese 4.0 to 4.5. Phosphorous .03. Sulphur .03. Silicon .6 to 1.0. Chromium 23.5 to 24.5. Nickel 3.5 to 4.0. Molybdenum 1.2 to 1.5. Balance essentially Iron.

Percent Carbon .55 to .65. Manganese 8.0 to 9.0. Phosphorous .04 max. Sulphur .04 max. Silicon .25 to .65. Chromium 21.25 to 22.75. Nitrogen .3 to .4. Balance essentially Iron (No Nickel).

Percent Carbon .47 to .58. Manganese 8 to 10. Phosphorous .04. Sulphur .04 to .09. Silicon .25 max. Chromium to 22. Nickel 3.25 to 4.50. Nitrogen .38 to .50.

Balance essentially Iron.

In general these alloys differ essentially in the relative proportions of nickel, silicon, manganese and nitrogen. These steels contain the iron essentially in austenitic form, since this form possesses a markedly higher hot strength than the ferritic form. To this end the nickel, manganese and nitrogen are present in the alloys to promote the austenitic structure, the manganese and nitrogen being present essentially as a substitute for the nickel. The silicon which is normally present as a residual constituent and promotes a ferritic structure is reduced in various alloys to discourage the formation of the ferrite. These austenitic valve steels have a melting point in the vicinity of 2550 F. and 2650 F.

The present invention is concerned basically with the application of a relatively hard, oxidation resistant, high hot strength alloy coating to the aforementioned austenitic steel valve surfaces which is a cobalt-chromium alloy including minor proportions of tungsten and carbon. A specific example of an alloy of this type for use in the present invention is as follows:

Percent Chromium 28 Tungsten 4 Carbon 1.0

Balance essentially cobalt.

For the purpose of the present invention the chromium may range from to the tungsten from 2 to 12%,

the carbon from .5 to 3%. The alloy may contain minor amounts of other elements such as silicon and residual elements.

In accordance with the method of the present invention, the valve part to be coated is first cleaned to remove forging scale, grease and the like. The cleaning may be accomplished by dipping the valve in a molten electrolytic caustic salt (such as the commercially available product known as Kolene) at a temperature of about 900 F. The valves may then be washed in water and then further cleaned by acid pickling. A suitable pickling bath is an aqueous solution containing about 2% hydrofluoric acid, 7% sulphuric acid, and 10% nitric acid. In some instances the valves may be adequately cleaned by a single degreasing treatment in a chlorinated solvent. Mechanical methods such as grit blasting, sand blasting, hydroblasting, etc., may be advantageously employed in some instances to supplement the chemical cleaning treatment.

After the valve is thoroughly cleaned, a relatively low melting point, hard, oxidation and wear resistant nickelchromium alloy including minor proportions of carbon and boron and/ or silicon is applied onto the valve surface in a layer of about 0.001 to 0.010 inch and preferably of about 0.002 to 0.004 inch thickness. The nickelchromium alloy is preferably applied by a metal spray technique utilizing a powder or an impregnated plastic wire in a spray gun as is well known in the art preferably onto a rapidly rotating valve whereby a uniform layer of the alloy of predetermined thickness is applied. A specific preferred composition of this alloy is as follows:

Percent Chromium 13.5 Boron 3.5 Silicon 4.5 Iron 4.5 Carbon 0.8

Balance essentially nickel.

- boron as indicated above, since the boron provides superior hot hardness and wear resistance. It is essential to the invention that the above nickel-chromium alloy have a melting point substantially below that of the valve steel base metal, as well as the cobalt-chromium alloy above referred to.

The melting point of the alloy may be in the range of about 1850 F. to 1950 F. and preferably about 1900 F., although alloys of this type having a melting temperature as high as 2000 F. may be satisfactorily used.

After application of the nickel-chromium alloy a layer of the above described cobalt-chromium alloy is applied over the aforementioned nickel-chromium alloy layer in a layer of about 0.005 to 0.020 inch and preferably of about 0.008 to 0.010 inch, preferably by a metal spray technique utilizing a cast rod or powder in a metal spray gun, as above described.

Next the coated valve is preferably fluxed by immersion in an aqueous potassium pentaborate solution at about 150 F. to 200 F. A solution containing 15 to 35 grams of potassium pentaborate per cubic centimeters of water at room temperature has been found satisfactory for this purpose.

The flux valve is then subjected to heating, preferably by induction heating, at the temperature of the melting point of the intermediate nickel-chromium alloy layer. In this process the nickel-chromium alloy layer is caused to melt and fuse and also to Wet the base metal of the valve and the cobalt-chromium layer, whereby the cobalt-chromium alloy layer is caused to bond firmly to the valve base metal through the nickel-chromium alloy layer.

The use of the fluxing step is not essential since the cobalt-chromium alloy layer will bond -to the valve base metal through the nickel-chromium layer whether or not the fluxing step is used, since the silicon and boron of the nickel-chromium alloy provide it with self-fluxing properties. However, the surface of the outer cobalt-chromium alloy layer will become oxidized during the heating step. In the event that the outer cobalt-chromium alloy layer should contain pores, which are desirably to be filled by the nickel-chromium alloy under layer during the melting thereof to provide a smooth surface finish, the oxidized surface would tend to restrict the flow of the melted undercoating into such pores and, accordingly, the fluxing step is advantageous in the production of a smooth surface finish. In some instances it is desirable to apply an additional nickel-chromium alloy layer over the cobalt-chro- [[IllllIIl alloy layer to insure that during the fusion or heating step the outer nickel-chromium alloy layer will fill any pores in the cobalt-chromium alloy layer and provide a smooth valve surface. This layer is also very thin, in the neighborhood of about 0.002 to 0.004 inch, although the layer may be 0.001 to 0.006 inch in thickness with satisfactory results. In this event the fiuxing step may be advantageously employed. After the composite layers are fused, it may be desirable to dip or otherwise apply a very thin layer of aluminum over the coated valve and subsequently fusing the aluminum to provide additional oxidation resistance.

In the process of the present invention the nickel-chromium alloy and the cobalt-chromium alloy layers are bonded to the base metal valve without softening and dimensionally distorting the valve base metal surfaces. Moreover, the coatings are applied smoothly in predetermined thicknesses whereby the resulting valve is of approximately the final desired dimension and little, if any, grinding or the like is necessary to form a finished valve.

Although the invention has been described in terms of an exhaust valve, it is obvious that the principles of the invention are applicable to other articles such as valve seat inserts, spark plug casings and the like, which are exposed to similar service conditions, and it will be understood that the scope of the invention is not to be limited thereby except as defined in the following claims.

We claim:

1. A process for providing an austenitic steel article With a hard, high hot strength and high wear resistant and oxidation resistant surface, the steps comprising applying in particulate form a first coating of a nickel-chromium alloy including minor proportions of silicon, boron, and carbon as essential constituents and having a melting point less than the softening temperature of said steel article over said surface; applying in particulate form a second coating of a cobalt-chromium alloy including minor proportions of tungsten and carbon as essential constituents having a melting point higher than that of the metal of said first coating over said first coating; and thereafter heating said first coating to at least its melting temperature for a time suflicient for said first coating to fuse and wet said second coating and said surface to bond said second coating to said surface through said first coating.

2. Claim 1 wherein the nickel base alloy consists essentially of 10 to 18% chromium, 2 to 5% boron, 2 to 5% silicon, to iron, 0.1 to 1.0% carbon and the balance substantially nickel and the cobalt base alloy consists essentially of 25 to 35 chromium, 2 to 12% tungsten, .5 to 3% carbon and the balance substantially cobalt.

3. A process for providing an austenitic steel article with a hard, high hot strength, high wear resistant and oxidation resistant surface, the steps comprising applying in particulate form a first coating in the range of 0.001 to 0.010 inch in thickness of a nickel-chromium alloy consisting essentially of to 18% chromium, 2 to 5% boron, 2 to 5% silicon, 0 to 5% iron, 0.1 to 1.0% carbon and the balance substantially nickel to a surface of said article; applying in particulate form a second coating of a thickness of 0.005 to 0.020 inch of a cobalt-chromium alloy consisting essentially of 25 to 35% chromium, 2 to 12% tungsten, .5 to 3% carbon and the balance substantially cobalt over said first coating; and thereafter heating said first coating to at least its melting temperature and substantially below the melting temperature of said metal article and said second coating for a time sufficient for said first coating to fuse and wet said metal article surface and said second coating and bond said second coating to said article through said first coating.

4. A process for providing an austenitic steel exhaust valve with a hard, high hot strength, high wear and oxidation resistant surface, the steps comprising spraying a first coating of a thickness of about 0.002 to 0.004 inch of a nickel-chromium alloy consisting essentially of about 10 to 18% chromium 2 to 5% boron, 2 to 5% silicon, 0 to 5% iron, 0.1 to 1.0% carbon and the balance substantially nickel onto a surface of said valve; spraw'ng a second coating of a thickness of about 0.008 to 0.010 inch of a cobalt-chromium alloy consisting essentially of about 25 to 35 chromium, 2 to 12% tungsten, 0.5 to 3 carbon and the balance substantially cobalt over said first coating; and thereafter induction heating said first coating to at least its melting point and substantially below the melting temperature of said exhaust valve for a time sufiicient for said first coating to fuse and bond said second coating to said valve through said first coating.

5. A process for providing an austenitic exhaust valve with a hard, high hot strength, high wear resistant and oxidation resistant surface, the steps comprising spraying a first coating of a thickness of about 0.002 to 0.004 inch of a nickel-chromium alloy consisting essentially of 10 to 18% chromium, 2 to 5% boron, 2 to 5% silicon, 0 to 5% iron, 0.1 to 1.0% carbon and the balance substantially nickel over a valve surface; spraying a second coating of a thickness of about 0.008 to 0.010 inch of a cobalt-chromium alloy consisting essentially of 25 to 35% chromium, 2 to 12% tungsten, 0.5 to 3% carbon and the balance substantially cobalt over said first coating; spraying a third thin coating of said nickel-chromium alloy over said second coating, immersing the coated valve in an aqueous potassium pentaborate solution and then induction heating said nickel-chromium alloy to at least its melting point and substantially below the melting temperature of said valve steel for a time sufficient for said first coating to fuse and bond said second coating to said valve through said first coating.

6. A process for providing an austenitic exhaust valve with a hard, high hot strength, high wear and oxidation resistant surface, the steps comprising spraying a first coating of a thickness of about 0.002 to 0.004 inch of a nickel-chromium alloy consisting essentially of about 13.5% chromium, 3.5% boron, 4.5% silicon, 4.5% iron, 0.8% carbon and the balance substantially nickel over a surface of said valve; spraying a second coating of a thickness of about 0.008 to 0.010 inch of a cobaltchromium alloy consisting essentially of about 28% chromium, 4% tungsten, 1% carbon and the balance substantially cobalt over said first coating; immersing the coated valve in an aqueous potassium pentaborate solution and then induction heating said nickel-chromium alloy to at least its melting point and substantially below the melting temperature of said article for a time sufiicient for said first coating to fuse and bond said second coating to said valve through said first coating.

7. A process for providing an austenitic exhaust valve with a hard, high hot strength, high wear and oxidation resistant surface, the steps comprising spraying a first coating of a thickness of about 0.002 to 0.004 inch of a nickel-chromium alloy consisting essentially of about 10 to 18% chromium, 2 to 5% boron, 2 to 5% silicon, 0 .to 5% iron, 0.1 to 1.0% carbon and the balance substantially nickel over a surface of said valve; spraying a second coating of a thickness of about 0.008 to 0.010 inch of a cobalt-chromium alloy consisting essentially of 25 to 35% chromium, 2 to 12% tungsten, .5 to 3% carbon and the balance substantially cobalt over said first coating; immersing the coated valve in an aqueous potassium pentaborate solution and then induction heating said nickel-chromium alloy to at least its melting point and substantially below the melting temperature of said valve steel for a time sufficient for said first coating to fuse and bond said second coating to said valve surface through said first coating.

8. A process for providing an austenitic steel exhaust valve with a hard, high hot strength, high wear and oxidation resistant surface, the steps comprising spraying a first coating of a thickness of about 0.002 to 0.004 inch of a nickel-chromium alloy consisting essentially of about 10 to 18% chromium, 2 to 5% boron, 2 to 5% silicon, 0 to 5% iron, 0.1 to 1.0% carbon and the balance substantially nickel over a surface of said valve; spraying a second coating of a thickness of about 0.008 to 0.010 inch of a cobalt-chromium alloy consisting essentially of about 25 to 35% chromium, 2 to 12% tungsten, 0.5 to 3% carbon and the balance substantially cobalt over said first coating; and thereafter induction heating said first coating from at least its melting point to about 2000 F. and substantially below the melting temperature of said exhaust valve for a time suflicient for said first coating to fuse and bond said second coating to said valve through said first coating.

References Cited in the file of this patent UNITED STATES PATENTS 2,390,452 Mudge Dec. 4, 1945 2,584,161 Scherer Feb. 5, 1952 2,903,375 Peras Sept. 8, 1959 10 2,955,958 Brown Oct. 11, 1960 FOREIGN PATENTS 574,903 Germany Apr. 22, 1933 635,891 Great Britain Apr. 19, 1950 

1. A PROCESS FOR PROVIDING AN AUSTENITIC STEEL ARTICLE WITH A HARD, HIGH HOT STRENGTH AND HIGH WEAR RESISTANT AND OXIDATION RESISTANT SURFACE, THE STEPS COMPRISING APPLYING IN PARTICULATE FORM A FIRST COATING OF A NICKEL-CHROMIUM ALLOY INCLUDING MINOR PROPORTIONS OF SILICON, BORON, AND CARBON AS ESSENTIAL CONSTITUENTS AND HAVING A MELTING POINT LESS THAN THE SOFTENING TEMPERATURE OF SAID STEEL ARTICLE OVER SAID SURFACE; APPLYING AN PARTICULATE FROM A SECOND COATING OF A COBALT-CHROMIUM ALLOY INCLUDING MINOR PROPORTIONS OF TUNGSTEN AND CARBON AS ESSENTIAL CONSTITUENTS HAVING A MELTING POINT HIGHER THAN THAT OF THE METAL OF SAID FIRST COATING OVER SAID FIRST COATING; AND THEREAFTER HEATING SAID FIRST COATING TO AT LEAST ITS MELTING TEMEPERATURE FOR A TIME SUFFICIENT FOR SAID FIRST COATING TO FUSE AND WET SAID SECOND COATING AND SAID SURFACE TO BOND SAID SECOND COATING TO SAID SURFACE THROUGH SAID FIRST COATING. 